1. Field of the Invention
This invention relates to an electronic device for sensing gas present in the environment.
2. Description of the Related Art Including Information Disclosed Under 37 C.F.R. .sctn..sctn.1.97 and 1.98
Numerous types of devices currently exist for sensing the presence of gas in environments. In particular, sensing devices for flammable gases such as methane or LPG for domestic use are very widespread. These devices generally consist of:
1) a gas presence sensor, usually based on semiconductor, catalytic or electrochemical technology, which provides as output an electrical signal proportional to the gas concentration in the air; PA0 2) an electronic circuit for processing the output signal from the sensor (a threshold comparator or a digital processing logic circuit); PA0 3) an electrical circuit for optical and/or acoustic alarm indication, which operates a buzzer or siren and/or lights a lamp or an LED light source.
Sometimes the device also comprises an electrical actuator circuit which acts on auxiliary members, such as solenoid valves for halting gas delivery, or comprises an electrical or electronic output circuit arranged to repeat the alarm indication at other supplementary indicator means.
However, devices of this type have numerous drawbacks, which it would be desirable to reduce or, better still, eliminate. In this respect, firstly during the production of such sensing devices, the sensors themselves are calibrated on the basis of the input/output specifications for the electronic signal processing components downstream of said sensors.
In this manner, the sensitivity and the threshold sensing level for the gas concentration in the environment, and declared on the data plate of the sensing devices produced, become standardized. Moreover, checking for possible faults (in the sense of phenomena which make it impossible to effect normal calibration) is carried out on a small number of devices at a time, as the fault indication is given independently by each sensing device. In addition, current national and international regulations on this subject, plus proper production procedures, both require an operational test to be carried out on all manufactured sensing devices in an environment in which a gas concentration equal to an operating threshold value is present.
It is apparent that, again in this case, the number of devices which can be checked is limited by the fact that each of them independently provides intervention and/or an alarm indication. According to known techniques, the device is often calibrated manually by operating a potentiometer or by selecting resistors with different resistance values, to be connected to the measurement sensor. Calibrating the device is therefore particularly delicate and critical and requires lengthy and costly manual operations, as the sensor has to be placed in environments of known gas concentration at controlled temperature.
Sometimes, when a digital processing circuit is provided, electronic calibration is carried out consisting of recording in a permanent memory the compensation factors for the values read by the sensor in the presence of the known gas concentration. However this operation is performed in an environment of known gas concentration and controlled temperature, and hence the calibration is dependent on the temperature of the atmosphere in which the gas is present, and to a certain extent it also depends on the percentage of moisture in the environment in which the device is installed. Finally, even in the case of electronic calibration the sensing devices must be calibrated one at a time. All this results in lengthy times for these operations, due also to the fact that the gas sensors require a stabilization time during calibration in a known atmosphere. Moreover the chambers containing measured gas must be washed periodically, the sensors must be preheated, and other costly and bulky production equipment has to be provided to ensure the necessary degree of compatibility between the measurements effected and the results awaited. Finally, possible operations involved in temperature-control or conditioning of the calibration and test environment have to be taken into account.